The primary goal of this research program is the development of an easily-usable, non-traumatic apparatus for the accurate assessment of body volume in normal and low birth weight infants. A secondary goal is the inclusion of an assessment of energy expenditure into the same apparatus. The method proposed for the volume determination incorporates the proven method of fluid displacement but utilizes a unique concept which allows the use of air as the working fluid instead of a more conventional liquid displacement approach. The addition of energy expenditure measurement capability relies on the use of an integral heat exchanger/calorimeter and on the ability to measure the heat contained in water vaporized by the infant in its normal respiration and thermal control physiology.